1. Field of the Invention
This invention relates to a waveform improving circuit for improving the waveform contour of a signal whose band is reduced such as, for example, a chrominance signal reproduced in a video tape recorder (VTR).
2. Description of the Prior Art
For a method of improving the waveform contour of a signal having its band reduced, refer to, for example, Laid-open Japanese Patent Application No. 59-89077. FIG. 12 is a block diagram showing a conventional circuit for this purpose. FIGS. 13(A) through (H) are diagrams showing the waveforms of signals of the circuit in FIG. 12. Referring to FIG. 12 and FIGS. 13(A) through (H), a color difference signal a whose band is reduced is inputted to a terminal 30 and differentiated by a differentiation circuit 31 to be a signal b. The signal b thus obtained is subjected to a full-wave rectification by a full-wave rectification circuit 32 to be a signal c. The signal c thus obtained is waveform-shaped by a waveform shaping circuit 33 to be a signal d. The input signal a is subjected to sample-hold in response to this signal d by a sample-hold circuit 34 thereby to obtain an output signal e. Referring to these diagrams, however, it can be pointed out that the output signal e is improved in the contour of its waveform but a deviation of time t is taken place at the edge portion thereof.
On the other hand, when the change in the level of the input signal a is small, the differential signal c is small, so that the control signal d cannot be generated. Thus the input signal is outputted with no improvement in the waveform contour. This means that deviation amount of the edge portion of the waveform contour of a signal depends on the waveform of that signal. Time interval t2 at which a sample-hold is made changes depending on an amount in change of the input signal. When the waveform of the input signal is changed largely, it is delayed and, on the other hand, when it is small in change, it is not delayed, thus resulting in variations in the timing of the edge of the waveform. From the standpoint of television image, colors which are small in change are shifted to the left thereby to cause a color divergence.